A conventional electron emitting device has employed a hot cathode for extracting electrons by heating a material such as tungsten (W) or the like to a high temperature. In recent years, a small electron emitting device of a cold cathode type has been attracting attention as an electron beam source to replace such an electron gun. Examples of this type of electron emitting device include that of a field emission type and an avalanche amplification type employing a pn and a Schottky junction.
In the electron emitting device of the field emission type, electrons are emitted from a cone-shaped emitter portion made of metals such as silicon (Si), molybdenum (Mo) or the like by applying a voltage to a gate electrode so as to apply an electric field. Thus, this type of electron emitting device has an advantage of being miniaturized using miniaturization techniques.
On the other hand, in the electron emitting device of the avalanche amplification type employing a semiconductor material, hot electrons are emitted from an emitter portion by applying reverse bias so as to cause avalanche amplification.
The characteristics required for a material for such an electron emitting device are as follows: 1) readily emitting electrons with a relatively small electric field, i.e., having a small electron affinity; 2) having a chemically stable surface for an emitter portion in order to maintain stable electron emission; 3) having excellent abrasion resistance and heat resistance.
In view of the above-mentioned points, in the field emission type device of the prior art, an amount of an emitted current has a large dependence on the shape of the emitter portion, thus making the production and the control thereof excessively difficult. In addition, it has a problem in surface stability of the used material. Furthermore, in this system, an individual device is an electron emission source at a point, and it is difficult to obtain electron emission flow in a plane shape.
In the avalanche amplification type, since it is generally necessary to apply a large current amount to the device, the device generates heat. Thus, the electron emission characteristic becomes unstable, or the life time of the device is shortened. Furthermore, in the avalanche amplification type, a cesium layer or the like is provided on the surface of the emitter portion so as to reduce a work function amount in the electron emission portion. However, since a material having a small work function such as cesium is chemically unstable, the state of the surface is not stable, i.e., the electron emission characteristic is not stable. As described above, the materials and the structures that have been conventionally used are not sufficient to provide the characteristics required for the electron emitting device.
On the other hand, diamond is a semiconductor material having a wide band gap (5.5 eV), and the characteristics thereof are suitable for a material for the electron emitting device due to its high hardness, abrasion resistance, high thermal conductivity and chemical inactivity. In addition, diamond can have a higher energy level of the conduction band edge than the energy level of vacuum by controlling the state of the surface thereof. In other words, diamond can have the state of negative electron affinity. More specifically, when electrons are injected to the conduction band of a diamond layer, the electrons are readily emitted. Furthermore, in general, diamond can be easily formed by a chemical vapor deposition (CVD) method using a gas containing carbon species and a hydrogen gas as source gas, and has an advantage in production thereof. However, since the Fermi level of a metal is significantly different from the energy level of the conduction band of diamond, it is not easy to supply electrons to the conduction band of the diamond simply by contacting an electrode with the diamond layer. A method or a mechanism for efficiently supplying electrons to the conduction band of the diamond has not been studied in detail, and an electron emitting device has not been realized so far that can supply electrons to the conduction band of the diamond and allow the electrons to be emitted.